Vacuum Tube vs Flat Panel

Efficiency

Vacuum tubes are much more efficient than flat panel collectors. Heat gained within the tubes is not so easily lost through the vacuum, while in a flat panel collector, heat can be lost as rapidly as it is gained, especially in high temperature applications.

Flat panel collectors perform poorly in low irradiation settings. When there's a lot of clouds, or the sun is on the horizon, the flat panel collectors retain little or no thermal energy because any heat slowly gained is quickly lost through the non-insulated components of the collector's surface.

In high temperature applications, such as home heating or absorption cooling, vacuum tube solar water collectors out perform flat panel collectors in every circumstance.

When it comes to flat panel solar collectors, the absorber area is nearly identical to the gross area. On vacuum tube collectors, the absorber area tends to be around only half of the gross are of the collector. When comparing the two through the SRCC efficiency equations, the vacuum tube collector will appear to have a much lower efficiency due to these gaps between the tubes and the manifold.

The gaps between the tubes in vacuum tube collectors are not as much of a problem as would be expected since the sun is only directly above in the sky for a short period of time. As the tubes passively track the sun as it moves across the sky through the day, more of the sunlight which passes between the tubes when perpendicular gets absorbed through the sides and backs of the tubes, rather than passing between the tubes. The transverse incident angle modifiers actually gain so much extra sunlight during the morning and evening that it greatly reduces the gap between the flat panel and vacuum tube collector overall efficiencies throughout the day.

There is a solution to the gaps between the tubes in a vacuum tube collector. Reflectors can be mounted behind the tubes, so that any sunlight not absorbed will reflect back towards the tubes and have a 2nd chance to be absorbed while passing through. This is especially effective when the sun is perfectly perpendicular to the collector, as this is when the gaps are most exposed. However, when the sun is at a high angle, the reflectors do not make any difference as the tubes themselves absorb all of the energy on the first pass from the passive tracking.

It is estimated that the reflectors provide an additional 50% extra sunlight to the tubes for the overall day. The theoretical maximum gain, based on the gaps in the Duda Solar collectors is about 76%. With reflectors, it is easy to conclude that vacuum tube collectors outperform flat panel collectors even at low ΔT between the collector and the ambient.

Cost

Flat panel collectors are more expensive than vacuum tube collectors since they contain a lot more expensive materials such as copper piping. It will not be uncommon for a flat panel collector to cost 50% or more than a vacuum tube collector of similar size.

Flat panel collectors also require a lot more fluid to fill all of the extra pipes, which means more antifreeze expenses when it comes time to changing out the fluids.

If space and not cost of the collector is the determining factor in choosing flat panel over vacuum tubes, it should be noted that a reflector set for a Duda Solar collector only costs an additional 8-10% towards the final cost of the vacuum tube collectors. These reflectors, which give around 50% more sunlight, make up for the gap in gross area efficiency, and still provide nearly the same or more heating power at low temperature applications, and of course a lot more heating power at high temperature applications.

Installation & Maintenance

Flat panel collectors come as one heavy piece. They must be carried up to the roof where they will be installed and mounted as one piece. Should any damage occur to the collector, the entire collector is rendered useless. A large hailstorm can bring death to a solar water heating system consisting of flat panel collectors, and will require full replacement to bring it back to efficient operation.

Duda Solar Vacuum Tube Collectors come in different pieces. The stand is put together and mounted to the roof, and then tubes are plugged into the collector. If a tube breaks from bad weather it can easily be replaced with a spare tube. Duda Energy provides extra tubes with each collector as insurance during shipment, and any extra tubes that make it to the customer can be kept as spares in the case of breakage during installation or use.

Although the life of a solar water heater is long, there will eventually come a time when it needs to replaced. However, vacuum tube collector parts can be replaced as needed. If some tubes go bad, they can be swapped. If the manifold gets corroded from poor heat transfer fluid quality, it can be swapped and the tubes kept. If something goes wrong in a flat panel collector, the entire unit will likely need to be replaced.

Installation of a vacuum tube collector is also much safer. The heat absorbing into the inside of the tubes does not rapidly transfer to the outside of the tubes. Therefore, a very hot tube can be picked up by hand and will feel cool to the touch. A flat panel collector in the sun will be hot and difficult to work on during the day.

Climate

Vacuum tube collectors hold up against high winds better than flat panel collectors since they have gaps between the tubes which allow for the wind to pass right over them. If the wind catches a flat panel collector just right, it could be sent flying through the air, resulting in damage to the roof it was mounted to and to the surrounding areas.

The question of snow comes up a lot. The argument of flat panel collectors is that when they are covered with snow, any light that passes through the snow to the collector will result in the glass heating up, and thus the snow melting. The collector can then resume full efficient solar collection.

A vacuum tube collector doesn't really need this heat loss feature. Snow falls between the tubes and does not pile up on the collector, allowing the collector to gather heat from the sun even while snow is falling from the sky. In the event that there is enough snow build-up to actually cover the tubes, the collector will continue providing heat from any sunlight that passes through the tubes, rather than dumping it back to the snow. Any light stopped by the snow, will convert to thermal energy and melt the snow from the tubes.